In a series of mutation experiments, using both X-rays and EMS, an extensive cytogenetic analysis will be made of both mutant and nonmutant treated chromosomes. The contribution of chromosomal rearrangement to lethality, sterility, and visible mutation will be assessed. The distribution of lethal mutations along the X chromosome will be determined by tests with several different short duplications that can be used to "cover" induced lethal mutants. Particular regions that appear to be free of induced lethal effects, or to mutate at an especially low rate, will be explored in the hope of finding evidence for the existence of persisting duplications in the normal genome. One such case is now known to exist in the 3C region, as we have recently found. Further, the difference between mutant and nonmutant rearrangement breakpoints will be investigated; is it a matter of exactly where, or how the chromosome is broken? Also, the detailed relationships of specific genes to particular salivary chromosome bands will be studied, as will the mutability of genes associated with large bands in comparison with that of genes associated with thin, delicate bands. In all experiments, the mutational consequences of treating germ cells at different stages of maturity will be compared (for both chromosomal and nonchromosomal mutants) by testing, separately, each successive brood of progeny produced after treatment.